Sodium hydroxide is incredibly soluble in fresh water. You can, if you want, make a solution that has more sodium hydroxide than water, but I'd probably try a 10% solution (1 part sodium hydroxide in 10 parts water, and be very careful to not do too much at once as it will boost the tank pH a lot, and alkalinity somewhat. You can always boost the concentration if that doesn't work.
Here's a recent thread:
Hello Randy,
i just received 500g of fisher scientific Sodium Hydroxide which i would like to mix with RO water so i can inject into aipstasia. Can you please tell me what portions of water to pellets i should use...
sincerely,
Robin